Implement confstr (unistd.h)

The implementation for confstr is straightforward. Most of the constants
return 1 on both musl and glibc. The only constant that doesn't is
required for Fish.

I also switched an #[unsafe(no_mangle)] from my last patch back
to a #[no_mangle] because we need to bump cbindgen. #[unsafe(no_mangle)]
is required for Rust 2024.

tests/Makefile

@@ -124,6 +124,7 @@ EXPECT_NAMES=\
 	unistd/access \
 	unistd/brk \
 	unistd/constants \
+	unistd/confstr \
 	unistd/dup \
 	unistd/exec \
 	unistd/fchdir \

tests/unistd/confstr.c

@@ -0,0 +1,34 @@
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#define CSPATHSZ 9
+#define BADSIZ 4
+
+int main(void) {
+    char actual[CSPATHSZ] = {0};
+    assert(confstr(_CS_PATH, actual, CSPATHSZ) == CSPATHSZ);
+    const char expected[] = "/usr/bin";
+    assert(strncmp(expected, actual, CSPATHSZ) == 0);
+
+    // The constants other than _CS_PATH just return an empty str (no support).
+    char empty[] = "";
+    assert(
+        confstr(
+            _CS_POSIX_V6_LP64_OFF64_LIBS,
+           empty,
+           0
+        ) == 1
+    );
+
+    // Buffers that are too small should return the expected size.
+    char small[BADSIZ] = {0};
+    assert(confstr(_CS_PATH, small, BADSIZ) == CSPATHSZ);
+
+    // Null buffer and zero length should return the expected size
+    // for the constant.
+    assert(confstr(_CS_PATH, NULL, 0) == CSPATHSZ);
+
+    return EXIT_SUCCESS;
+}